Stop getting addons channel from @storybook/addons

We're seeing issues where the addons channel exported by
@storybook/addons is different from the globally available channel
window.__STORYBOOK_ADDONS_CHANNEL__.

In the past, we used the global variable but switched to importing it in
05328d7. I can't recall the details there but seems safe to
move back.

I have manually checked that this fixes things for one of our customer.
The symptoms were that all stories appeared to be rendered but they
didn't actually render on screen. Switching to the global channel did
the trick.

src/register.js

@@ -1,5 +1,3 @@
-import { addons } from '@storybook/addons';
-
 const { SB_ROOT_ELEMENT_SELECTOR } = require('./constants');
 
 const time = window.happoTime || {
@@ -163,21 +161,20 @@ window.happo.nextExample = async () => {
         console.error('Failed to invoke afterScreenshot hook', e);
       }
     }
-    const channel = addons.getChannel();
-    channel.emit('setCurrentStory', {
+    window.__STORYBOOK_ADDONS_CHANNEL__.emit('setCurrentStory', {
       kind: component,
       story: variant,
       storyId,
     });
     await new Promise((resolve) => time.originalSetTimeout(resolve, 0));
     if (theme && themeSwitcher) {
-      await themeSwitcher(theme, channel);
+      await themeSwitcher(theme, window.__STORYBOOK_ADDONS_CHANNEL__);
     }
     await waitForSomeContent(rootElement);
     if (/sb-show-errordisplay/.test(document.body.className)) {
       // It's possible that the error is from unmounting the previous story. We
       // can try re-rendering in this case.
-      channel.emit('forceReRender');
+      window.__STORYBOOK_ADDONS_CHANNEL__.emit('forceReRender');
       await waitForSomeContent(rootElement);
     }
     if (beforeScreenshot && typeof beforeScreenshot === 'function') {